Why did political and territorial domination of colonial areas become necessary for the European powers?

For a few reasons. The high standards of living acquired over the years by the West, especially during the industrial growth of the 19th century, made many people want more things. . . more tasteful, popular, and expensive things. It was during the second half of the 19th century that workers were drinking coffee and tea in the morning, a sure sign of class raise. This 19th century rise was accompanied by another rise in population, not to mention more child-conscious parents. This high class style of thinking was felt in different ways throughout all of Europe. To put it metaphorically:

The 19th and early 20th century world was a garden of growing flowers. For a while there had just been two or three flowers getting all the sunlight, Britain, France and the Dutch. Now the flower of Germany had grown and it's stem was big and strong; it wanted the sunlight just like all the other flowers. As I said before, the leaves on these flowers were hungrier and more numerous, and only the flowers that got the most light from the colonial sun got to grow tall enough to become one of the Great Flowers, a title of significant honor and repute throughout the garden. Aside from Germany, Japan's Flower was growing taller, ever since the American Rain Storm, which had once been in the sky shining on Britain, had fell down on it and helped it bloom. The garden was growing higher and wider than it had ever grown before, because every plant needed cheap and ethnically inferior sunlight to quench the maw of its leaves.